If you always use a mono speaker, just make sure all your inputs are not panned, and set the width of the stereo inputs to 0%.
In that case you can connect your speaker only to the Left or Right output, both will be mono.
If you want to use the stereo output and the mono output simultanious, you can create a Matrix mix, send the main to the matrix with 0% width and send it to one of the mix outputs.